Tsunami and Brighton.

There has been a good deal of public fear-mongering in articles since the Dec 26th Indian Ocean earthquake. While the possibility of a tsunami is ever-present, what we need to do in the here-and-now is actually quite simple.

In a Distant Tsunami alert, follow the Civil Defence instructions in the back of the Yellow Pages.

In a Near Tsunami situation, there is no plan yet and no available refuges. Don your life jackets.

Let’s examine these two very different situations more closely.

First of all, tsunami can originate either from a Distant or a Near source. Their effects are quite different.

Distant Tsunami
We know quite a lot about Distant tsunami and their historical effects, and there is a Pacific Ocean tsunami warning system in place, thanks to US efforts after the 1960 tsunami which had disastrous effects in Hawaii.

Distant tsunami arrive after an earthquake along an American (South or North) fault line. They take at least 12 hours to cross the Pacific, and thus there is plenty of time to organize and carry out the existing Civil Defence plan. This advises a retreat from the coast of several kilometers, or a climb to at least 35 metres above sea level.

The historical pattern of distant tsunami effects on our immediate coastlines is also quite predictable. Tsunami are affected by two factors: the sea-bed profiles offshore, and the coastline shape itself. Anything offshore (such as reefs – artificial or natural, islands), can act to dissipate energy. Other features such as submarine canyons can act to focus it. Both effects have been observed in Canterbury.

Generally speaking, the broad beaches of Pegasus Bay have seen only minor wave run-ups from Distant tsunami. These have been contained by existing seawalls (e.g. by North Beach Surf Club and at Central Brighton), by dunes elsewhere, or else the waves have simply been too small to be noticeable – their energy was directed elsewhere.

However, in any confined area – the Waimak mouth, the Estuary, Sumner, and all of the Peninsula bays plus Lyttelton Harbour, the tsunami waves tend to be focused and intensified. Houses were carried away at Okains and Le Bons Bay in the 1868 tsunami, and 3-5 metre waves are consistently recorded for Lyttelton during most significant tsunami.

The NOAA website and tsunami search engine here has a useful summary of historical tsunami effects locally. Search for coordinates -43 North to -46 South, and from 172 West to 179 East, and run-up country “New Zealand” to pick up the East Coast around Canterbury.

But as noted, there is at least time to prepare for such an event. We cannot realistically save buildings in the bays, but we can save ourselves. And as was the case for the recent North Island floods, these events are insurable, so that the loss of buildings and houses is not necessarily an economic problem. There is time to collect personal objects of value.

In short, a Distant Tsunami is not something to be feared. Prepared for, insured for, planned for, trained for - yes. On notification, the drill is – follow the Civil Defence instructions in the back of the Yellow Pages. And don’t rush – there is time, and you need to be sensible.